“Lithium-ion batteries can be used in small Electronic devices as well as larger applications such as electric vehicles and power grids. They can easily meet the requirements of various sizes, voltages and shapes. However, this breadth of application means that battery manufacturers have to purchase and maintain test solutions for each battery type, resulting in huge related capital investment, which directly accounts for 20% of the final cost of the battery.
Lithium-ion batteries can be used in small electronic devices as well as larger applications such as electric vehicles and power grids. They can easily meet the requirements of various sizes, voltages and shapes. However, this breadth of application means that battery manufacturers have to purchase and maintain test solutions for each battery type, resulting in huge related capital investment, which directly accounts for 20% of the final cost of the battery.
Obviously, there is a need for a cost-effective, multi-range battery test solution that can handle different voltages, capacities and dimensions. This article introduces the advantages of the digital control loop battery tester, and provides a flexible and cost-effective battery test design example.
Advantages of digital control loop
The main function of the battery tester is to control and monitor the charge and discharge of the battery. Figure 1 shows the block diagram of the switching battery tester. The control part can be realized in analog or digital mode. In an analog implementation, a pulse width modulation (PWM) controller can adjust the output voltage or current flowing through the high-voltage and low-voltage power supplies. The protection circuit is integrated in the PWM controller. The constant current and constant voltage feedback loop drives the reference input of the PWM controller to precisely control the output current and voltage. A 16 digital-to-analog converter (DAC) connected to the feedback controller sets the output current and voltage. Finally, a precision 16-bit analog-to-digital converter (ADC) monitors the battery voltage and current.
Figure 1: Block diagram of battery tester
In the digital implementation, the microcontroller (MCU) performs all the functions in the red box in Figure 1. With the help of C2000™ real-time control MCU, 16-bit PWM can be generated and its comparator can be used to implement the protection algorithm. The MCU adjusts the current and voltage controllers through the data fed back by the ADC. Because the controller is in the digital domain, this architecture does not require a precision 16-bit DAC. A 12-bit on-chip ADC can achieve control accuracy of less than ±0.05%, which is sufficient for cost-optimized battery test systems. However, if you want to achieve a control accuracy of ±0.01%, then the solution requires an external 16bit feedback accuracy. The digital solution can easily achieve accuracy and flexibility; the performance and cost difference depends only on your choice to use external 16 in the feedback Bit ADC is also an internal 12-bit ADC. In this scheme, the efficient use of MCU can save more than 30% of material costs.
Current test equipment is designed for specific battery types. Larger batteries require higher current, so the battery tester has multiple channels connected in parallel. But if battery manufacturers are producing small batteries with lower current requirements, they usually use testers optimized for lower current levels, while leaving high-current battery testers in an idle state. Using a tester that can test both small and large batteries at the same time can reduce the redundancy of such equipment and help reduce the overall cost of battery production. Digital control loops increase the flexibility of using software to test large or small batteries, while analog solutions require hardware changes.
With the development and progress of battery technology, battery manufacturers require new functions and test methods. Using control loops in software makes it easier for test equipment manufacturers to provide other test functions.
Design a multi-range battery tester
The digital control reference design for cost-optimized battery test systems uses multiple independent control, low-current battery tester channels connected in parallel to meet the needs of different levels of high-current battery testers. With simple software changes, the reference design can be configured for multiphase operation, as shown in Figure 2. In a multi-phase configuration, each phase uses an independent constant current loop connected in parallel. In the constant voltage mode, a constant voltage loop is connected to all constant current loops to ensure current balance. Therefore, the same test environment can provide multiple output current ranges.
Figure 2: Feedback controller in a multiphase configuration
The block diagram of the reference design is shown in Figure 3. TMS320F280049 MCU can control up to 8 independent channels. It can generate high-resolution 16-bit PWM for the synchronous buck power stage and execute subroutines for the current and voltage control loops. The INA821 instrumentation amplifier senses current, and the TLV07 operational amplifier senses voltage. The ADC of the external ADS131M08 and the on-chip ADC of the C2000 both convert current and voltage signals into digital information. According to the 16bit ADC signal fed back, a control accuracy better than ±0.01% can be obtained. For cost-optimized systems, ADS131M08 can be removed based on feedback, and the on-chip 12-bit ADC can achieve control accuracy of less than ±0.05%.
The realization of multiple feedback controllers can realize a smooth transition from constant current to constant voltage, in which the internal loop is always in constant current mode. When the constant voltage mode condition is detected, the output of the constant voltage circuit will be connected to the constant current circuit.
Figure 3: Digital control loop battery tester
The digital architecture can achieve high accuracy, high current, high speed and flexibility without the need to invest a lot of money in battery test equipment. You no longer need to invest in testers with multiple architectures for different current levels, so that high-current devices are no longer idle when testing low-current applications.
With the help of TI’s digital control reference design, the overall system cost can be saved by investing in low-current battery test equipment, thereby improving the ability and flexibility to test multiple current ranges without affecting accuracy.